1. Field of the Invention
The present invention relates to an electrical hand-held power tool for a rotatable tool and including a safety clutch that at critical operational conditions disconnects the electric motor of the power tool from the power transmitting chain. In particular, the present invention relates to a hammer drill with a safety clutch.
2. Description of the Prior Art
Electrical hand-held power tools such as, e.g., hammer drills, are driven by a rapidly rotating electric motor that, therefore, is provided with a capability to store a large amount of rotational energy due to its angular momentum. To prevent an unpermissible deflection of a hand-operated electrical power tool at critical operational conditions, in particular, when the working tool becomes locked, the drive power must be quickly disconnected, or the power transmission chain between the electric motor and the working tool should be broken sufficiently quickly. In electrical power tools with a large power, the driving power cannot be disconnected sufficiently rapidly without using additional means.
For breaking the power transmission chain, clutches, which are arranged in the transmission chain between the electric motor and the working tool, are used. Usually, the clutches are formed as safety clutches that break the transmission chain when a transmitted torque exceeds a predetermined threshold. The conventionally used clutches are formed as pure mechanical slip clutches such as disclosed in German Publication DE 40 36912, or as electrically controlled, electromagnetic clutches such as disclosed, e.g., in European publication EP 486 842. For power transmission in an engaged condition, coupling elements, which are primarily arranged radially outside of the two coaxial shafts and are frictionally connected with each other, are used.
European publication EP 694 706 discloses a slip clutch that has forcelocking coupling means which is arranged radially outside of two coaxial hollow shafts engaged in each other and which consists of two coupling elements associated with respective shafts and which slide relative to each other when a predetermined adjustable static friction force is exceeded.
In electrical power tools with an electric motor arranged transverse to the working tool axis, a bevel gear set is primarily used for transmitting torque between the axis of the electric motor rotor and the working tool axis. The bevel gear set includes a bevel pinion supported on the rotor shaft, as disclosed in German Utility Model DM-92 08 407, or is arranged parallel to the rotor shaft.
For reducing the dimension of the power tool in a direction transverse to the working tool axis, U.S. Pat. No. 4,346,767 discloses an arrangement of a safety clutch in the power transmission chain. The clutch is offset axially and is structurally integrated in the intermediate stage of the gear set.
The drawbacks of the in-line arranged, safety clutches consist in that they need additional bearings for their support, which should be provided in the power transmitting chain, and an increase of the volume of the power tool housing. Providing more than two support points for a shaft results even with a small displacement, in a disadvantageous tension in the drive chain.
Accordingly, the object the present invention is to provide an electrical hand-held power tool in which the safety clutch is supported directly on a rapidly rotatable rotor shaft, with a minimum increase of the axial dimension of the tool housing.
Another object of the present invention is to provide an electrical hand-held power tool with a safety clutch that would provide for an active braking of the rotor shaft.
These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing an electrical hand-held power tool in which the pinion shaft is formed as a hollow shaft, and an inner bearing for rotatably supporting the pinion shaft is provided in its interior, and the rotor shaft has its end received in the interior of the pinion shaft and supported against the inner bearing.
The safety clutch is provided between the pinion and rotor shafts and provides for their joint rotation during a normal operation and insures at a critical condition their rotation relative to each other. The rotor shaft engages in the hollow pinion shaft supported for rotation by the inner bearing. The inner bearing is formed as a slide bearing that is centered with respect to the common, with the pinion and rotor shaft, axis by a central ball. The inner bearing is provided with sufficiently long axial slide surfaces which insure a tilt-free absorbtion of the transverse forces applied to the pinion. A radial outer bearing, which is provided in the region of engagement of the rotor shaft in the pinion shaft, supports the rotor shaft and, via the inner bearing, the pinion shaft for rotation in the power tool housing. A rotor bearing provided at an end of the rotor shaft remote from the engagement region absorbs the transverse forces.
Advantageously, the safety clutch is formed as an electromagnetic clutch in which at least the adjacent radially end surfaces of the rotor disc of the clutch supported on the pinion shaft for joint rotation therewith, and of the anchor disc of the clutch rotatably supported on the rotor shaft for joint rotation therewith, are formed of a ferromagnetic material. During a normal operation, a magnetic flux, which is produced by the magnetic coil, causes attraction of the rotor and anchor discs to each other, providing for their form-or forcelocking connection.
Advantageously, the hollow cylindrical magnetic coil, which is mounted on a non-ferromagnetic end plate of the power tool housing, is surrounded at its three free sides by the rotor disc for transmitting magnetic flux thereto. The outer radial bearing, which supports the pinion and the rotor shafts, is arranged within the magnetic coil. As a result of such an arrangement, the outer bearing, the rotor disc, and the magnetic coil are nested within each other in a limited axial region. As a result, only a very insignificant lengthening of the housing is required for the integration of the safety clutch.
During interruption of the power transmission by the electromagnetic clutch, the braked pinion shaft comes relatively rapidly to a standstill as result of fiction forces generated by the working tool and in the gear set. The connection of the relatively light anchor disc with the rotor shaft only insignificantly increases its rotational energy, which result, as usual, in the runout of the electric motor. Advantageously, only after the electric motor comes to a standstill, the electromagnetic clutch is energized again, which prevents an inadvertent power transmission to the working tool.
Advantageously, a friction lining is provided on a surface of a pot-shaped clutch cover which faces the adjacent end surface of the anchor disc. At a critical condition, a spring, which is arranged within the anchor disc and which becomes preloaded by the electromagnetic clutch, biases the anchor disc and friction lining into a frictional contact with each other. The anchor disc provides for braking of the electric motor that comes to standstill, with the rotational energy of the motor being dissipated as a result.
The magnetic coil can be secured to the end plate of the housing and be pre-assembled together with the rotor disc and the pinion shaft. In the next step, the rotor shaft, together with the anchor disc and the housing or clutch cover, which also serves as a dust protector, is pushed into the pinion shaft.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to is construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings.